<book id="b1">
  <part id="P1">
    <CHAPTER>
      <foo />  -&lt;unknown&gt;
      <section id="s1">
        <section id="s11" />
      </section>
      <section id="s2" />
      <section id="s3">
        <para id="p31" />
      </section>
    </CHAPTER>
    <CHAPTER>
      <foo />  -&lt;unknown&gt;
      <section id="s4" />
      <section id="s5" />
    </CHAPTER>
  </part>
</book>